1. Field of the Invention
The present invention relates to a heat treatment technique for performing a heating process on a substrate including a semiconductor wafer, a glass substrate for a liquid crystal display device and the like, with the substrate placed on a heating plate. More particularly, the invention relates to a heat treatment apparatus and a heat treatment method for exposing a substrate placed on a heating plate to a flash of light emitted from a flash lamp to perform a heating process on the substrate.
2. Description of the Background Art
Conventionally, a lamp annealer employing a halogen lamp has been typically used in the step of activating ions in a semiconductor wafer after ion implantation. Such a lamp annealer carries out the activation of ions in the semiconductor wafer by heating (or annealing) the semiconductor wafer to a temperature of, for example, about 1000° C. to about 1100° C. Such a heat treatment apparatus utilizes the energy of light emitted from the halogen lamp to raise the temperature of a substrate at a rate of about hundreds of degrees per second.
In recent years, with the increasing degree of integration of semiconductor devices, it has been desired to provide a shallower junction as the gate length decreases. It has turned out, however, that even the execution of the process of activating ions in a semiconductor wafer by the use of the above-mentioned lamp annealer which raises the temperature of the semiconductor wafer at a rate of about hundreds of degrees per second produces a phenomenon in which the ions of boron, phosphorus and the like implanted in the semiconductor wafer are diffused deeply by heat. The occurrence of such a phenomenon causes the depth of the junction to exceed a required level, giving rise to an apprehension about a hindrance to good device formation.
To solve the problem, there has been proposed a technique for exposing the surface of a semiconductor wafer to a flash of light by using a xenon flash lamp to raise the temperature of only the surface of the semiconductor wafer implanted with ions in an extremely short time (several milliseconds or less). This technique is disclosed, for example, in U.S. Pat. Nos. 6,998,580 and 6,936,797. The xenon flash lamp has a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamp is shorter than that of light emitted from the conventional halogen lamp, and approximately coincides with a basic absorption band of a silicon semiconductor wafer. It is therefore possible to rapidly raise the temperature of the semiconductor wafer, with a small amount of light transmitted through the semiconductor wafer, when the semiconductor wafer is exposed to a flash of light emitted from the xenon flash lamp. Also, it has turned out that a flash of light emitted in an extremely short time of several milliseconds or less can achieve a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time by using the xenon flash lamp allows the execution of only the ion activation without deeply diffusing the ions.
Unfortunately, it has turned out that, when such a heat treatment apparatus employing the xenon flash lamp performs a heating process on a plurality of (e.g., 25) semiconductor wafers in a lot (or batch) in succession, the first several wafers in the lot have a sheet resistance lower than that of the remaining wafers after the heating process, i.e., the temperature of the first several wafers during the heating process is higher than that of the remaining wafers. In particular, the temperature of the first one wafer in the lot during the heating process is extremely high. Thus, there arises a problem such that the conventional technique fails to uniformly heat-treat all of the semiconductor wafers in a lot.